Automotive vehicles have lighting systems for the rear of the vehicle that include lamps that function as tail lights and/or brake lights. Tail lamps produce red light, and are typically wired such that they illuminate whenever the vehicle's front lights are illuminated. Brake lamps are wired such that they illuminate when a driver depresses the brake pedal. The tail lamps may be combined with the brake lamps, or separate from them. When combined, the lamps produce brighter red light for the brake light function, and dimmer red light for the tail light function. The tail and brake light functions may be produced separately or by a dual-intensity lamp.
Creating a brake light and a tail light function for Light Emitting Diode (LED) automotive tail lamps that use the same LEDs for a tail mode and a brake mode requires that the LEDs be driven “ON” to a lesser degree for the tail mode than for the brake mode. The partially “ON” tail mode can be achieved by principles of Pulse Width Modulation (PWM), wherein the LEDs are “ON” for less than 10% of the time. The PWM timing can be implemented by pulsing the LED driver ON/OFF in an open loop fashion from a timing source. Unfortunately, when the LED driver is a switchmode power supply (SMPS), there can be variability in the time it takes to start-up and shut-down the power supply, especially if the SMPS has soft start or is commanded from its soft start control circuits. The variation in the start-up/shut-down time of the power supply impacts the duty cycle performance of a resultant PWM waveform. Specifically, SMPS circuits take time to start-up and shut-down. The start-up and shut-down time changes in response to external conditions such as ambient temperature, input voltage, and load.
A constant PWM duty cycle of an output of the power supply is desirable for combination tail mode and brake mode LEDs. Open loop PWM of the power supply circuits from a soft-start control input allows the duty cycle to drift due to varying start-up and shut-down delay times in the power circuits. A drifting duty cycle causes a drifting perception and measurement of the brightness of the tail mode LEDs.
U.S. Pat. No. 8,120,273 to Edwards discloses a light control system and method for controlling a PWM duty cycle of the light control system.